Though it was discovered in 1958, it is only recently that melatonin, a hormone produced by the pineal gland, has been intensively studied. In 1993 melatonin made its first appearance as a consumer product hailed for its sleep inducing effect. Melatonin supplements are being investigated as a potential treatment to readjust biorhythms and alter sleep cycles in people with jet lag, the blind, and nightshift workers. It is also being explored for its potential in the treatment and prevention of various diseases.
Melatonin works to maintain the body's circadian rhythms, regulating such human functions as body temperature, dream sleep, the sleep/wake cycle, and the secretion of cortisol. The cue for the release of this hormone begins when the eyes first register the onset of darkness. The body requires darkness to produce melatonin. Thus, levels of this hormone are high at night and low during the day. This chemical is not produced during daytime naps, as light suppresses its production.
Melatonin originates from tryptophan, an essential amino acid which is obtained through the diet. During the day the body converts tryptophan into serotonin (a chemical involved in mood). Serotonin is then converted to melatonin (usually at night). Large doses of tryptophan increase melatonin levels and induce sleep.
Unlike other hormones, melatonin does not need a receptor to enter a cell membrane and can permeate any part of a cell. It is absorbed directly into the bloodstream, other tissues and body fluids. Infants begin producing melatonin about the fourth day after birth. Prior to birth, melatonin passes through the placenta to the infant. This chemical is also present in breast milk.
Melatonin is found in some plants, mammals, birds, reptiles, and amoebas, regulating activities such as mating, migration, and hibernation. Some animals exhibit higher melatonin levels in fall and winter months when the number of daylight hours decreases. This same pattern has been noted in women but not in men. Melatonin levels peak during childhood and gradually decrease throughout adolescence and adulthood. Declining levels of this hormone trigger the onset of puberty in children and menopause in women. Melatonin levels are lowest in older adults, which may explain the prevalence of insomnia in this population.
Sufferers of Seasonal Affective Disorder (SAD) may have high daytime levels of melatonin and consequently abnormal circadian rhythms. SAD is a condition marked by depression associated with the declining level of daytime light during the winter months. Exposure to light, which can alter the timing of melatonin secretion, is found to be therapeutic for this condition.
Melatonin is being considered as a potential treatment for breast cancer because of its ability to limit the amount of estrogen entering cells. High exposure to the hormone estrogen is believed to be the cause of many types of breast cancer. In tests on rats, melatonin has been found to decrease the size of malignant mammary tumors by 50%.1 The effects of melatonin have also been studied when used in conjunction with tamoxifen, another anti-estrogen chemical which is a possible treatment for breast cancer. These studies indicate tamoxifen is 100 times more effective against cancer cells that have previously been treated with melatonin. Research also indicates that patients with high blood levels of melatonin have greater success with chemotherapy.
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